North America has vast deposits of tar sands, which are mixtures of viscous hydrocarbons and sand. Some of these deposits are consolidated (sand stone) while others are unconsolidated and disintegrate upon heating. A minor percentage of the deposits are at or close to the surface, and are mined by removing any overburden, and then physically removing the tar sands to plants in which the viscous hydrocarbons are separated from the sand. The adhesive nature of the tar sands, and their abrasiveness, tend to make the operations difficult and expensive, particularly in the upkeep of equipment. In spite of the difficulties, commercial operations are currently being conducted in Canada.
However, over 80% of the tar sands deposits are situated well under the surface of the earth, far enough below so that removal of the overburden is not practical. In many locations, there are beds of tar sands 100 feet and more in thickness, situated 300 feet or more below the surface. There has been no commercial exploitation of this huge reserve of hydrocarbons, which are larger than the known oil reserves of the Persian Gulf.
Workers in the field have approached the problem in various ways. The most logical prior art suggestions known by us are made in the Walker U.S. Pat. No. 3,858,654--Jan. 7, 1975, and the Redford U.S. Pat. No. 3,951,457--Apr. 20, 1976. In those patents, a well is sunk through the overburden into near the bottom of the tar sands deposit, and the well is cemented to the overburden. Hot aqueus alkaline fluid is directed against the tar sands to heat it to the point where the hydrocarbons become sufficiently liquid so that they can be forced up the well to a recovery system where the hydrocarbons are separated from the hot aqueous fluid. During mining, the cavity is maintained at a pressure high enough to support the overbruden, using a non-condensable gas to maintain the pressure. The injected aqueous fluid is maintained at about 180.degree. to 200.degree. F. to obtain a tar sands temperature of 160.degree. F., preferably near 180.degree. F.
The methods suggested by these patents have not been commercialized for a number of reasons. The recovery of the hydrocarbon values will be difficult to accomplish in a single decanter, as suggested in the patents, because the specific gravity of the heavy hydrocarbons is very near that of water. In addition, the patents disclose no effective provision for preventing roof collapse either during mining or after completion of the operation.
It is the principal object of this invention to provide a method of hydraulic mining of unconsolidated tar sands at depths unsuitable for strip mining, which is both energy efficient, and which provides means for preventing collapse of the cavity during, and after completion of, the mining.